Hilary Duff's son Luca has some interesting ideas when it comes to the naming of his unborn sister.

The Younger actress made her first public appearance since announcing she is expecting her second child – a baby daughter with singer boyfriend Matthew Koma – on Monday's Late Late Show with James Corden.

Naturally, the star's pregnancy was a hot topic and Hilary opened up about breaking the news to her six-year-old son Luca, who she shares with former husband Mike Comrie.

"The beginning was a little" he was surprised, and now he's warming up to the idea," she shared with host James Corden. "I think it's a girl, but he told his friends at school the other day that he's having a sister that's part red-butt monkey, so we'll see. We never know."

When James probed whether the couple had thought of any names for their little girl, the 30-year-old revealed it's Luca who is taking the lead with a slightly unusual, albeit bold, suggestion.

"We're up for any kind of suggestions, but Luca thinks Cafont Croissant is, like, the coolest name," she laughed. "When I just refer it as "Cafont,' and I'm like, "That would be so cool, nobody at school's gonna be named Cafont,' and he's like, "Croissant. Cafont Croissant.' It like goes together, there's no separating the two."

James and actor Sam Richardson, who also appeared on the show, claimed to be in favour of the quirky name choice, leaving the Stranger singer to question: "It's pretty strong, right? You guys are convincing me!"

Hilary announced she is pregnant on Friday with a touching post shared to her Instagram account.

Underneath an image of the A Cinderella Story star enjoying a romantic embrace with Michael, she wrote: "Guess what guys! @matthewkoma and I made a little princess of our own and we couldn't be more excited!!!!!"